Preheat your oven to 375°F and bring a large pot of salted water to boil. Cook jumbo pasta shells according to package directions until al dente (usually 9-11 minutes), then drain carefully and set aside to cool slightly so they're easier to handle.

While pasta cooks, whisk together pumpkin puree, ricotta cheese, egg, minced garlic, Italian seasoning, nutmeg, black pepper, and salt in a large mixing bowl. Fold in three-quarters of the mozzarella and all the parmesan cheese, stirring until the mixture is smooth and creamy.

Spread a thin layer of marinara sauce across the bottom of a 9x13-inch baking dish. Begin filling each cooled pasta shell with approximately 2 tablespoons of the pumpkin mixture, then arrange them seam-side down in the dish.

Pour the remaining marinara sauce over the filled shells, then drizzle vegetable broth around the edges of the dish. Sprinkle reserved mozzarella cheese and fresh sage on top, then cover with foil and bake for 30 minutes.

Remove foil and bake uncovered for an additional 10 minutes until the cheese is bubbling and golden tender pumpkin stuffed shells are heated through completely. Let cool for 5 minutes before serving to allow the sauce to set slightly and prevent spillage.